The New VarEQ Phono for 78rpm Records

Hi!

After the last Variable EQ phono stage for 78rpm records I always wanted to build one with switchable LCR EQ. Here it is.

Bernd Hemmen and Wolf von Langa with whom I will set up a system at the upcoming High End show again, came up with the idea to do 78rpm demos similar to the  one at the ETF in Berlin. A good reason to develop a new variable EQ preamp for such an event.

This time with D3a tubes and based on the same fully transformer coupled circuit as my D3a LCR RIAA phono stages.

Only mono this time and with separately adjustable bass turnover frequencies and treble roll off.

The last implementation had too many settings with 12 positions for each. This time I reduced to just 6 settings each.

The two D3a tubes are placed at the front and part of the chokes and transformers of the circuit are placed on top under transformer covers.

The power supply is completely separate:

Visit us at the High End fair and join us for a journey through the history of Jazz on 78rpm!

Best regards


Thomas

The Ultimate Line Preamplifier, Part 7

Hi!

In the last post about the ultimate line preamp I showed the work in progress on the latest build. Now it is finally finished.

Unlike the last three builds this one will stay in my system.

Finally I get to enjoy this preamp for myself!

It has the same color combination as my ultimate phono stage.

This combination will stay in my system, as my main demo preamp.

It will also be on demo at the upcoming High End fair in Munich.

Here we see 4 Hytron VT25 glowing away in the preamp:

The line stage together with the matching ERC8020 differential phono:

It has three inputs, of course it can also be built with more inputs if needed.

I already wrote about the sound in previous posts about this line stage so I am not going to bore you with more superlatives.

Together with the EC8020 differential phono this represents my ultimate statement in preampfification.

Come to the High End fair in May to listen to this preamp.

Stay tuned for more info about the system which will be shown in Munich.

Best regards

Thomas

Tube of the Month : The UX112A

Hi!

After the presentation of the rather modern tube last month, I will take you back all the way to the 1920ies today with the presentation of a very early triode, the UX 112-A.

This tube was introduced as detector and amplifier triode. It was meant as an improved version of the UX201A.

The UX112A shares the same UX4 socket and pinout with the UX201A as shown on the left. They have almost the same amplification factor but the 112A has about twice the transconductance which results in half the plate resistance. So definitely a major improvement in specifications. This was achieved by changing the thoriated tungsten filament to an oxide coated one. Filament voltage and current stayed the same (5V, 0.25A). In most applications the UX112A could simply drop in to a socket meant for the UX201A. See the Cunningham data sheet for all technical parameters of this tube. The UX201A gained some popularity recently and is used in line preamplifiers. I also experimented with it and consider it as a nice sound processing device. I have not really done much with the UX112A yet, but as mentioned a circuit could easily be designed such that it accepts both tubes which would enable to compare the specific differences between the thoriated tungsten and oxide coated filaments. The 112A definitely has nice plate curves:

While the UX201A was made in large numbers and can still be found rather easily, Less UX112A tubes are around nowadays, but still not impossible to find. Here some 112As from my stock.

Here we have a beautiful Cunningham CX-112-A.

Nice shiny nickel plate.

Unlike the UX201A the glass bulb is not almost entirely covered with getter so the internals are usually visible from one side while the other is completely mirrored:

Some more views:

RCA Radiotron UX-112-A:

Some detail close ups:

Another Radiotron with different construction:

Later Versions by RCA / Cunningham:

These dropped the 'UX' prefix.

Nicely sealed box which allowed testing of the tube without breaking of the seal.

These already have the more modern ST-shape.

Later versions dropped the 3 digit naming and were simply dubbed 12A. Here an example from Sylvania:

National Union 12A:

Some close ups of the plate structure:

A JAN (Joint Army Navy) NU 12A/112A made for the military:

An older National Union in globe shape:

Beautiful coloured box and gorgeous globe shape tube.

Nicely engraved base.

The top:

Side views:

The oxide coated filament of the UX112A only shows a rather dim glow:

With the room lights off:

That's all for this beautiful triode. I hope you enjoyed the journey back to the 1920ies.

Best regards

Thomas